The present invention relates to the use of a temporary or fugitive alloying element to promote a phase transformation in a metal. Hydrogen is of particular interest, particularly with respect to titanium alloys, because it has significant effects on some metal systems and may be removed from the metal after treatment.
Hydrogen has been previously used to modify the properties of titanium and its alloys. It has been used to embrittle titanium to facilitate its comminution by mechanical means to form titanium metal powders. In such techniques hydrogen is diffused into the titanium at elevated temperatures, the metal is cooled and brittle titanium hydride formed. The brittle material is then fractured to form a powder. The powder may then have the hydrogen removed or a compact may be formed of the hydrided material which is then dehydrided, U.S. Pat. No. 4,219,357 to Yolton et al.
Hydrogen also has the effect of increasing the high temperature ductility of titanium alloys. This characteristic has been used to facilitate the hot working of titanium alloys. Hydrogen is introduced to the alloy which is then subjected to high temperature forming techniques such as forging. The presence of hydrogen allows significantly more deformation of the metal without cracking or other detrimental effects, U.S. Pat. No. 2,892,742 to Zwicker et al.
Hydrogen has also been used as a temporary alloying element in an attempt to alter the microstructure and properties of titanium alloys. In such applications, hydrogen is diffused into the titanium alloys, the alloys cooled to room temperatures and then heated to remove the hydrogen. The effect of the temperature of introducing and removing the hydrogen on the structure and properties of titanium alloys was investigated W. R. Kerr et al. "Hydrogen as an Alloying Element in Titanium (Hydrovac)," Titanium '80 Science and Technology (1980) p. 2477.
The present invention is directed to the treatment of metal castings subsequent to the casting operation. It is particularly concerned with metal castings using metals or alloys which undergo a solid state allotropic transformation on cooling from elevated temperature, particularly the Group IVB elements and their alloys, including titanium.
In the production of Group IVB element alloy castings such as titanium, it is well known that certain structural imperfections may limit the suitability of the material for its intended applications. This is particularly important in highly stressed, critical applications such as gas turbine engine and other heat engine components, airframe, space vehicle and missile components, and orthopedic implant devices, such as hip joints and knee protheses. These limitations have become increasingly important in recent years because precision castings are being specified more frequently for critical applications because of their intrinsic cost advantage compared to competitive methods of manufacture.
Voids are one general type of imperfection which can exist in Group IVB element castings as a result of microshrinkage, cavity shrinkage, and other effects resulting from solidification. It is well known to those skilled in the art that this type of imperfection can be eliminated by hot isostatic pressing (HIP).
Another type of imperfection which has traditionally limited the utility of Group IVB element castings is unsatisfactory chemical compositional control in surface regions that are in contact with the mold material during solidification. Because of the relatively high chemical reactivity of Group IVB alloys, surface imperfections such as oxygen enrichment, contamination, and alloy depletion effects may be encountered. Within recent years, methods to circumvent this type of difficulty have become generally known. The techniques include the use of more refractory mold materials to limit the extent of surface interaction, and the use of specialized chemical milling treatments to remove desired amounts of surface material in a reproducible manner after casting, and thereby achieve dimensional accuracy in the final part.
A third type of limitation of Group IVB element castings arises because of the influence of the material's allotropic transformation on the casting's solidification history. This results in a microstructure which is coarser than that achieved with deformation processing operations such as forging. Coarse microstructures, in turn, usually are associated with reduced dynamic low temperature properties such as fatigue strength.
With reference to FIGS. 1 and 2, the microstructural coarsening in an unalloyed Group IVB metal (FIG. 1) or a Group IVB based alloy such as Ti-6Al-4V (FIG. 2) arises in the following way. On cooling from the liquid, the material solidifies to form a solid of the high temperature body center cubic (BCC) allotrope, which is referred to herein as beta. On further cooling in the mold, the material reaches the beta transformation (beta transus) temperature (T.sub.T in FIG. 1) where all or part of the beta transforms to the low temperature, hexagonal close packed (HCP) allotrope, which is referred to herein as alpha. In the case of the pure metal (FIG. 1), the as-cast microstructure consists entirely of alpha ("transformed beta") platelets, the orientation of which relate to certain crystallographic planes of the prior beta phase, and the size of which relates to both the cooling time through the transformation temperature and the subsequent cooling rate. In the case of an alloy such as Ti-6Al-4V, (FIG. 2) the material exhibits a coarse two phase microstructure of alpha ("transformed beta") plus beta, because the example alloy contains sufficient alloying element content to stabilize some fraction of the beta to room temperature. In either case, the alpha which has formed is a relatively coarse transformation product of the high temperature beta phase, (hereafter "transformed beta") and it is the coarseness of the alpha which generally limits the mechanical properties of the material, particularly the low temperature dynamic properties such as fatigue strength.
Broadly speaking, there are two conventional ways to address the problem of microstructure coarseness. One is to subject the material to a deformation processing operation such as forging to "break down" and refine the structure. This method has the further advantage that an equiaxed so-called "primary alpha" phase, which traditionally has been unobtainable in a cast structure, can be formed during deformation processing, thereby permitting the achievement of microstructures which are particularly desirable for fatigue limited applications. Unfortunately, forging is an energy, capital and raw material intensive operation. In addition, it is not readily applicable to components designed to be produced as cast net shapes.
A second approach is to heat treat castings above the beta transus temperature (e.g., at temperature T.sub.1 in FIGS. 1 and 2) to "solution treat" the material and return it to an all beta structure, and then to cool the article at a relatively rapid rate using either a stream of inert gas or a hyperbaric inert gas chamber. Optionally, this may be followed with one or more intermediate temperature aging treatments. Relatively fine microstructures can be obtained in this way because it is possible to obtain faster cooling rates using an appropriately designed heat treatment furnace than is generally achievable within the mold during and after solidification of the casting.
It is known that both of these approaches may be used to improve the properties of cast materials. As indicated above, castings are characterized by a coarse alpha (transformed beta) microstructure which, except for certain specialized applications, is generally improved by such treatments. Except for certain specialized (e.g., creep limited) applications, thermal treatment above the beta transus temperature is not generally applicable to wrought Group IVB alloys such as titanium alloys because it tends to eliminate the fatigue resistant, recrystallized "primary alpha" microstructure formed during deformation processing and return the material to a transformed beta microstructure.
Unfortunately, heat treatment of Group IVB alloy castings above the beta transus temperature has certain limitations:
(1) There is a tendency to induce beta grain growth which has the undesirable effect of increasing the grain size of the material.
(2) The use of relatively high processing temperatures, which must be performed in a vacuum or inert gas environment, subjects the material to an increased risk of interstitial surface contamination. The extent of this risk tends to increase with increased solutioning temperature.
(3) Due to simple heat transfer considerations, there are section size limitations on the ability to achieve a rapid cooling rate.
(4) The use of rapid cooling rates subjects the material to significant dimensional changes and the risk of distortion and cracking.
The present invention relates to the use of a "catalytic" or "fugitive" solute to induce a phase transformation in a metal and in that manner refine the microstructure without the complications of forging or the limitations of conventional heat treatments. As will be set out in greater detail in following portions of the specification, the solute that has the effect of lowering a transformation temperature is diffused into the metal when it is below a transformation temperature. The presence of the solute causes the transformation and the removal of the solute reverses the transformation.
By example, a removable solute, such as hydrogen, may be used as a temporary alloying element in Group IVB metals and their alloys as a means to promote the alpha to beta or the alpha plus beta to beta phase transformation, and the reverse reactions, under controlled conditions. In this manner microstructural refinement can be obtained under substantially isothermal processing conditions, at temperatures which are significantly below those required for traditional solution treatment and quenching operations.
Such a process is schematically illustrated in FIG. 3 which shows the effect of a solute element which stabilizes the high temperature beta allotrope to lower temperatures. In its simplest form: (1) the material is heated to temperature T.sub.2, which can be several hundred degrees below T.sub.T and T.sub.1 ; (2) the solute is introduced into the material such that the composition moves along line OP of FIG. 3, thereby isothermally solution treating it into the beta phase field; (3) the solute is rapidly removed from the material (reversibly along line PO, for example), to isothermally "quench" the material; and (4) the material is cooled to room temperature using conventional means.